There are a number of well-known methods of projecting an image onto a screen where it can be viewed. Probably the earliest is the pin hole camera. This employs an opaque panel with a small hole through which light passes. Light reflected from a subject passes through the pin hole and strikes the screen where an image is formed. An advantage of this design is that the projected image is in focus at all distances between the screen, the pin hole, and the subject being projected; a disadvantage is its extreme optical efficiency. Of all the light that is reflected from the subject only a very small portion passes through the pin hole. This results in a very dim projected image.
A similar method of projecting an image replaces the pin hole with a lens. In this scheme light reflected from the subject passes through a lens and strikes the screen where the image is formed. The advantages are greatly improved optical efficiency. The lens collects far more of the reflected light than the pin hole of the previous scheme. The major disadvantage is that the positioning of the subject, the lens, and the projection screen must be set according to the well-known lens formula. If the positions are not correct then the image formed on the screen will be out of focus.
Another method of projecting an image involves sequential scanning. The image is generated by scanning a beam across the screen. If the scanning process is fast enough the eye will see a steady image. Early televisions produced an image by mechanically scanning a light beam across the screen in raster fashion. The intensity of the light beam was modulated to produce the image. The current television system uses a cathode ray tube (CRT) that has an electron beam that sequentially scans a screen of phosphorescent material. The phosphorescent screen emits light when struck by the electron beam. The image is produced by modulating the intensity of the electron beam.
The major advantage of the sequential scanning systems is that the image can be transmitted long distances electronically. A major disadvantage is that the image resolution is limited by the speed at which each pixel that makes up the image can be refreshed. The image must be refreshed at least 25 to 30 frames per second for television and 60 to 80 frames per second for computer or the human eye will see flicker in the generated image.
An image that has 1024 pixels by 1296 pixels and is refreshed at a rate of 80 Hz has a pixel clock rate of at least 106 MHz. This is a common set of parameters used in computers today. As the number of pixels increases the pixel clock rate must increase as well. This places a limit on the resolution that is obtainable with the current technology. Further, as the pixel speed increases the amount of energy delivered to each pixel on the screen decreases thereby making the image dimmer.
It is possible to create larger images by using multiple CRTs to create an image that is tiled together. The image on a CRT is not geometrically precise due to the difficulty in accurately positioning the electron beam. The tiled images do not line up well. This leaves gaps and distortion at the boundaries.